Originally published on php-to-js.learnio.dev — part of the PHP to JavaScript course. View this lesson on the original site.

Pick `const` first, `let` when the value must change

You already declare variables all day in PHP. JavaScript does the same job with different syntax: no $, and a keyword that signals whether the name can be reassigned.

PHP habit

<?php
$name = 'Alice';
$count = 0;
$count = 1;

// One syntax for everything — readers cannot see reassignment intent from the keyword

JavaScript equivalent

const name = 'Alice';   // default: this name will not point elsewhere
let count = 0;          // this name will change
count = 1;
// name = 'Bob';        // TypeError: assignment to constant variable

PHP	JavaScript	What changes
`$name = 'Alice'`	`const name = 'Alice'`	Express “won’t rebind” at declaration
`$count = 0; $count++`	`let count = 0; count++`	Express “will rebind” with `let`
No direct equivalent	`var x = 1`	Legacy; avoid in new code

Mapping: `const` is not `final` on objects

In PHP you can still mutate an array or object held in a variable:

<?php
$user = ['name' => 'Alice'];
$user['name'] = 'Bob';   // fine
// $user = [];            // also fine in PHP — full reassignment allowed

const user = { name: 'Alice' };
user.name = 'Bob';    // fine — mutating the object
// user = {};         // TypeError — cannot rebind the const

const in JavaScript is closer to “this variable name always refers to this binding” than to freezing the whole structure.

Why `var` bites PHP developers

PHP’s $ in a loop body does not leak the way var leaks across a function:

<?php
for ($i = 0; $i < 3; $i++) {
    // $i is scoped to the loop in practice for most scripts
}

for (var i = 0; i < 3; i++) {
  // i still exists here — function scope, not block scope
}
console.log(i); // 3 — often a surprise

for (let j = 0; j < 3; j++) {
  // block-scoped — closer to what you expect from modern PHP style guides
}
// console.log(j); // ReferenceError

flowchart LR
  PHP["PHP: one `$` syntax"] --> JSconst["JS: const by default"]
  JSconst --> JSlet["let when reassignment needed"]
  JSlet --> Avoid["never var in new code"]

Try it

Use the amber Try it panel below. Click Run.

const role = 'PHP developer';
let lessonsCompleted = 0;
lessonsCompleted += 1;
console.log(role, lessonsCompleted);

Same mental model as PHP variables, with stricter signals in the keywords.

From PHP to JavaScript — JavaScript Syntax Foundations for PHP Devs

Pick const first, let when the value must change